The present invention relates to a system and a method for microlithographic writing and inspection on photosensitive substrates, and specially printing and inspection of patterns with extremely high precision, such as photomasks for semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. The terms writing and printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared to extreme UV.
A system and method for microlithographic writing of a substrate is previously known from e.g. EP 0 467 076 by the same applicant. In general such a system for microlithographic writing comprises a light source, such as a laser, a modulator to produce the desired pattern to be written, the modulator being controlled according to input pattern data, an acousto-optic deflector driven by a sweep frequency signal to scan the beam over the substrate according to a set of data indicating consecutive positions of the beam, and a lens to focus the beam before it reach the substrate. Further, the substrate is arranged on an object table, and the table (stage) is controlled by a servo system to be moved in a direction perpendicular to the scanning direction of the beam.
However, a problem with such known writing systems are that the table does not always perform a linear motion, and it is also possible that the table movement is not perpendicular to the scanning direction, but is made in another oblique angle.
A solution to this problem has been proposed by Whitney in U.S. Pat. No. 4,541,712, where the data to control the modulator is delayed in accordance with an offset measured for the substrate table. Hence, in this case the problem is handled by means of a timing control. However, this method is not feasible to use in all systems. It is further a problem with this known method that due to the delay function extra time has to be added at the beginning and the end of each sweep, whereby the time efficiency of the process is deteriorated. As a consequence, a less number of resolved pattern features per scan could be obtained as well.
It is further a related problem with the prior art that it is difficult to provide a reliable, accurate and effective translation from the input beam position data to an output sweep frequency signal to be used by the acousto-optic modulator to correctly direct the beams towards the substrate. This translation has heretofore normally been provided by a series of components, comprising a digital to analog converter (DAC) and a voltage controlled oscillator (VCO). These components, and especially the VCO, are non-linear, and are also sensible for changes in temperature, disturbances from other electronic components etc. Further, the VCO has an inherent analog xe2x80x9cinertiaxe2x80x9d preventing fast changes. Due to these and other problems with the translation from the input beam position data to an output sweep frequency signal the control of the deflector is deteriorated as well.
Raster-scanning inspection systems have a similar structure, and does hence experience similar problems.
It is therefore an object of the present invention to provide a system and a method in which the above mentioned problems of the prior art are solved or at least alleviated.
This object is achieved with a system and a method according to the appended claims.
According to a first aspect of the invention a laser scanning system for microlithographic writing or inspection of patterns on a photosensitive substrate is provided. The system comprises a laser light source generating at least one laser light beam, a computer-controlled light modulator controlled according to input pattern data, a lens to focus the light beam from the light source before it reaches the substrate, and a substrate support table to support the substrate. During the writing operation, the at least one beam is deflected across a region of the substrate surface by an acousto-optic deflector driven by a sweep frequency signal according to a set of data indicating consecutive positions of the beam on the substrate, and the substrate is moved in an oblique angel, and preferably perpendicular, to the direction of deflection to reposition it for exposure during the next stroke of the beams. Further, the system comprises at least one sensor measuring the extent of substrate offset in the direction of deflection, means for modifying the position data or the feeding of the data to correspond to laterally displaced scans and a control unit for controlling the reading out of the data to the deflector in dependence of the offset measured by the detector to compensate said offset. The laser source and the modulator may be integrated in one unit. Further, the means for modifying the data preferably comprises means for generating different sets of data. Hence, the offset compensation is achieved by controlling the deflector instead of the modulator.
Thus, the compensation for the measured offset is preferably not made by timing control, but rather by adding delays to the input data, or by choosing one of several possible sets of input position data, having different properties. Hereby, no extra compensation time is needed at the beginning and the end of each sweep, whereby the time is used more efficiently and the possible number of resolved pattern features per scan is improved.
According to a preferred embodiment of the invention, the laser scanning system comprises several data storage means for storing the different sets of data, and the control unit comprising a selector, selecting one of the data storage means data to read out to the deflector in dependence of the offset measured by the detector to compensate said offset. Alternatively the control unit comprises an adder to modify the position data in accordance with the measured offset, preferably in real-time, thus generating modified data to read out to the deflector to compensate said offset.
According to another aspect of the invention, a laser scanning system for microlithographic writing or inspection of patterns on a photosensitive substrate is provided. The system comprises a laser light source generating at least one laser light beam, a computer-controlled light modulator controlled according to input pattern data, a lens to focus the light beam from the light source before it reaches the substrate, and a substrate support table to support the substrate. During the writing operation, the at least one beam is deflected across a region of the substrate surface by an acousto-optic deflector driven by a sweep frequency signal according to a set of data indicating consecutive positions of the beam on the substrate, and the substrate is moved in an oblique angel, and preferably perpendicular, to the direction of deflection to reposition it for exposure during the next stroke of the beams. Further, the system comprises at least one sensor measuring the extent of substrate offset in the direction of deflection, and a direct digital synthesis (DDS) unit for generation of the initiation of the chirp, the DDS unit in turn being controlled by input data indicating the start of the sweep.
The DDS has a very suitable performance for this application, since it could perform the translation directly from input data to an output frequency signal, and thus replace several other components normally used. The DDS is further very stable, and is not as affected by disturbances in the environment as other components. Still further, the translation in the DDS is essentially linear, and the response time to changing data is very short.
Most preferably, however, said two aspects of the invention is used as a combination.
Further, the system preferably comprises a first high-frequency generator generating a first high-frequency signal and a mixer for mixing, i.e. adding or subtracting, the drive signal with the DDS to the high-frequency signal. Hereby, a low-frequency DDS, which is much cheaper and also more reliable and accurate, could be used, at the same time as the output frequency could still be maintained in a suitable range for driving an acousto-optic device in the writing or inspection process, typically 100-250 MHz.
It is further preferred that the high-frequency generator and the DDS unit are supplied with synchronised clock signals, and most preferably clock signals originating from the same clock. Hereby, synchronisation in the translation process could be maintained.
Still further, the system further comprises a second high-frequency generator generating a second high-frequency signal and a mixer for mixing the drive signal from the DDS, after the addition to the first high-frequency signal, from the second high-frequency signal. Hereby the relative range of the output frequency could be increased.
It is also preferred that the system comprises at least one unit for frequency multiplication, e.g. a mixer for mixing the input signal with itself, and this mixer preferably being placed between the mixers for mixing with the first and second high-frequency signals. This also increases the absolute range of the output frequency.
According to a preferred embodiment of the invention the computer-controlled light modulator and the DDS unit are further supplied with synchronised clock signals, and most preferably clock signals originating from the same clock. Thanks to this, the modulator and the deflector could be maintained in synchronisation throughout the entire writing or inspection process. Hereby uncertainty due to timing uncertainty between data and scan is avoided.
The invention also makes it possible to obtain two or more RF-signals with controlled phase-difference. Such phase-controlled signals could be used to extend the useful bandwidth of an acousto-optic deflector by so-called phased array driving. This could be accomplished by using at least two DDS units controlled by different input data in the system. Hereby, the input data to the channels have a small computed difference that creates a phase-difference. The two DDS units are preferably phase-synchronised at regular intervals and the same RF signals are used for up and down converting in the two units. In this way it is possible to create two signals with an extremely accurate phase-coherence but still having an arbitrary accurately controlled phase-difference. It could however also be obtained by a single DDS, a signal splitter and one or more phase-modulators as is well known in the RF-technology.
According to still another aspect of the invention a method for microlithographic writing with a laser scanning system of patterns on a photosensitive substrate is provided, the system comprising a laser light source, a computer-controlled light modulator controlled according to input pattern data, and a lens to focus the light beam from the light source before it reaches the substrate. During the writing operation, the at least one beam is deflected across a region of the substrate surface by an acousto-optic deflector driven by a sweep frequency signal according to a set of data indicating consecutive positions of the beam on the substrate, and the substrate is moved in an oblique angel, and preferably perpendicular, to the direction of deflection to reposition it for exposure during the next stroke of the beams; and the extent of substrate offset,in the direction of deflection is measured. Further, the method comprises the steps of generating sets of the position data corresponding to laterally displaced scans, and selecting one of said sets of position data to read out to the deflector in dependence of the offset measured by the detector to compensate said offset.
According to still another aspect of the invention, a method for microlithographic writing with a laser scanning system of patterns on a photosensitive substrate is provided, the system comprising a laser light source, a computer-controlled light modulator controlled according to input pattern data, and a lens to focus the light beam from the light source before it reaches the substrate. During the writing operation, the at least one beam is deflected across a region of the substrate surface by an acousto-optic deflector driven by a sweep frequency signal according to a set of data indicating consecutive positions of the beam on the substrate, and the substrate is moved in an oblique angel, and preferably perpendicular, to the direction of deflection to reposition it for exposure during the next stroke of the beams; and the extent of substrate offset in the direction of deflection is measured. Further, the sweep frequency drive signal is generated by a direct digital synthesis (DDS) unit, which in turn is controlled by input data indicating the start of the sweep.
Most preferably the two methods above are used in combination.